1. Field of the Invention
This invention relates to the field of precision axial solenoids and particularly to narrow width, high energy solenoids utilized in printing Braille characters on paper.
2. Discussion of Prior Art
Tubular solenoids are well known for various printing and actuation functions. A specific application of such solenoids is for the imprinting of dots on paper forming the well known Braille pattern in order to provide reading material for the visually handicapped. In such applications, a plurality of solenoids are arranged on a movable carriage where the carriage is capable of movement in both the x and y direction over a shoot of paper. During energization, the tip of the solenoid having a rounded surface thereon will impact the paper to be printed. The rounded tip deforms the paper which backed up by a platen either of hard rubber or a matching depression. The impact of the solenoid plunger on the paper crushes a dimple in the paper which can later be felt by a visually handicapped individual. However, as can be imagined, the time necessary to scan a sheet of paper in both the x and y direction and controllably print the desired. Braille characters thereon is extensive resulting in a relatively high cost for Braille printing materials.
An improvement in the Braille printing process is known as "interpoint" Braille in which two types of solenoids and two types of corresponding platens are utilized. One group solenoids, as noted above, has a rounded end which sandwiches the paper between the solenoid's rounded end and a depression in a corresponding platen. However the other group of solenoids have a hemispherical recess in the end of the solenoid which compresses the paper between the recess and a corresponding projection on the platen. Accordingly, depending upon which solenoid and platen combination is used, a bump can be made in the paper to extend towards the front of the paper or the back of the paper. Consequently, during a single pass over the paper, it can be printed in Braille on both sides thus resulting in a substantial decrease in the cost of printing such materials.
As can be appreciated, it would be desirable to dispense with both carriage systems so less time is spent scanning over the page. A system which utilizes 168 separate solenoids and has no moving carriage is currently being developed to print Braille on both sides of a sheet of paper from a single array of solenoids as the paper makes a single pass. An obvious requirement of such a device is a high energy solenoid capable of reliably deforming various weights of paper and, due to the number of solenoids, must be relatively narrow in diameter.
The spacing between dots on one side of the paper is on the order of 2.5 mm or 0.098". The dots have a width of about 1.25 mm or 0.047". Since a dot on one side of the paper cannot be co-located with a dot on the other side of the paper (in which a deformed dot at best would be provided), the location of the dots on one side of the paper is offset in the x and y directions by approximately 1.25 mm or 0.047" so that the dots on the front of the sheet of paper are "interpointed" with respect to the dots on the back of the sheet of paper. In view of the critical spacing and the need for the end of the solenoid to interact with its corresponding platen recess or projection, it is extremely important that the solenoid be highly accurate in its impact so that the position of the dot does not interfere with adjacent tints.
Conventional tubular solenoids utilize an electric coil wound around a plastic (generally nylon) bobbin. The central aperture in this bobbin is the area through which the plunger moves and is guided between its energized and deenergized state. In practice, the winding tension of coil wire on the bobbin varies between coil winding machines resulting in variations in the actual internal diameter of the plunger passageway. Moreover, as a solenoid is used, especially in the printing field, the coil generates heat which serves to expand the bobbin material which in turn narrows the plunger passageway.
Because of the winding tension variations and the possibility of bobbin expansion resulting in a narrowing of a passageway, the design clearance between the bobbin and the plunger must be maintained relatively high so as to insure that under the worst conditions mere is no binding of the plunger during operation. As a result of design for this worst case situation, conventional tubular solenoids have relatively poor guiding action and are not sufficiently accurate for the printing of Braille characters in particular interpoint Braille.
Furthermore, the close spacing of solenoids used in the printing of Braille characters, and particularly with interpoint Braille, creates difficulties in inserting and removing an individual solenoid from an array of solenoids without disassembling the entire array.